Beta barium borate (.beta.-BaB.sub.2 O.sub.4 ; BBO) is a non-linear optical material that is a low temperature form of barium borate. In single crystal form, BBO has been shown to be useful in a broad range of applications for modulation and frequency conversion of laser radiation. BBO has been shown to be particularly useful as a second harmonic generator or "doubler" for laser systems to obtain a different frequency radiation from a laser having a fundamental frequency. BBO is valuable in such laser systems because the material is an efficient harmonic generator, has a high optical damage threshold, has a large birefringence, and has good mechanical and thermal properties. In addition, BBO has a wide transmission region and, moreover, is essentially transparent to ultraviolet light having wavelengths as short as 190 nm.
However, it is not possible at present to grow single crystals of BBO of sufficient size and quality for large-scale applications. Consequently, the use of BBO crystals has been limited.
Standard melt growth techniques for the manufacture of suitable BBO crystals are precluded by a destructive solid-state phase transition near 925.degree. C. A metastable melt growth technique has been described in the Journal of Crystal Growth 106, BETA BARIUM BORATE SINGLE CRYSTAL GROWN BY A DIRECT CZOCHRALSKI METHOD by K. Itoh and F. Marumo, pp 728-731, dated 1990 (Itoh, et al.). However, due to the inherently large thermal gradients applied to the growing crystals, such a technique is likely to have shortcomings in producing large BBO crystals of high optical quality.
BBO can also be crystallized from solution below the phase transition temperature. This requires a solution to dissolve barium borate and to supersaturate BBO upon cooling to cause crystals of BBO to nucleate and grow or, alternatively, to cause a "seed" crystal set in contact with the solution to grow in size.
The most widely-reported solution growth technique involves high-temperature solution growth using sodium oxide, Na.sub.2 O, as a solvent and seeded growth at the surface of the solution. The use of this technique has been described in the Journal of Crystal Growth 89, GROWTH AND CHARACTERIZATION OF LOW TEMPERATURE PHASE BARIUM METABORATE CRYSTALS by L. K. Cheng, W. Bosenberg and C. L. Tang, pp. 553-559, dated 1988 (Cheng, et al.) and the Journal of Crystal Growth 97, SOLUTION GROWTH OF BARIUM METABORATE CRYSTALS BY TOP SEEDING by R. S. Feigelson, R. J. Raymakers and R. K. Route, pp. 352-366, dated 1989 (Feigelson, et al.). However, as noted by Itoh, et al., Cheng et al. and Feigelson et al., the resulting BBO crystals tend to suffer from the presence of optical inhomogeneities due to inclusion of growth solution. This is particularly troublesome if large optical elements for commercial applications are desired and inclusion cannot be controlled.
Immersion seeded growth techniques using Na.sub.2 O as a solvent have also been reported. The use of this technique has been described in the Journal of Crystal Growth 79, FLUX GROWTH 0F LARGE SINGLE CRYSTALS OF LOW TEMPERATURE PHASE BARIUM METABORATE by A. Jiang, F. Cheng, Q. Lin, Z. Cheng and Y. Zheng, pp. 963-969, dated 1986 (Jiang, et al.) and U.S. Patent No. 4,793,894 to J. Jacco and G. Loiacono (Jacco, et al.). However, due to the inherent surface energy anisotropies using this technique, the resulting BBO crystals take the form of thin needles or sheets which do not have any practical or useful dimensions, especially for large-scale applications.
Solution growth techniques using pure sodium chloride, NaCl, as a solvent have also been reported. The use of this technique has been described in U.S. Patent No. 4,931,133 to D. Gualtieri and B. Chai (Gualtieri, et al.), the Journal of Crystal Growth 97, GROWTH OF .beta.-BARIUM BORATE FROM NaCl-Na.sub.2 O SOLUTIONS by D. Gualtieri and B. Chai, pp. 613-616, dated 1989 (Gualtieri, et al. II) and the Journal of Crystal Growth 97, STUDIES ON FLUX SYSTEMS FOR THE SINGLE CRYSTAL GROWTH OF .beta.-BaB.sub.2 O.sub.4 by Q. Huang and J. Liang, pp. 720-724, dated 1989 (Huang, et al.). However, both Gualtieri, et al. and Gualtieri, et al. II report that pure NaCl is not suitable as a solvent for crystal growth of BBO for optical applications because of the inherently undesirable shape of the resulting BBO crystals, spurious nucleation, and uncontrollable solvent volatilization. Huang, et al. also describes solvent volatilization problems with the use of a pure NaCl solvent.
Consequently, there is a need to develop a method to grow single crystals of BBO of sufficient size and quality for large-scale applications. Further, there is a need to develop a solution growth technique that avoids spurious nucleation and minimizes solvent volatilization and that results in BBO crystals having minimum inclusions and desirable shapes with practical and useful dimensions.